This invention relates to chemical treatment of waste waters and, more particularly, to systems for chemically treating waste waters from metal treatment processes.
The waste water from various metal treatment processes, such as plating, cleaning, pickling, anodizing and the like, contain polluting constituents, such as hexavalent chromium and cyanides, which must be removed or chemically modified before the water can be safely disposed. Such waste waters are usually treated with reagents which chemically destroy the noxious constituents. For instance, waste waters from a metal plating process containing hexavalent chromium are treated with a reducing reagent and an acid under conditions whereby the hexavalent chromium is reduced to trivalent chromium. Waste waters from an anodizing process containing cyanides are treated with a oxidizing agent and a base under conditions whereby the cyanides are oxidized to cyanates.
Systems previously used for this purpose have included a collection or storage tank for the waste water and a reaction tank into which the waste water is transferred for chemical treatment. In these systems, the chemical reagents added to the waste water have been added into the reaction tank from supply containers either by gravity feed or by separate pumps for each reagent and the reaction tank includes a mixing or stirring means for dispersing the reagents throughout the waste water. When gravity feed is used, the containers of hazardous reagents are located above the reaction tank and usually overhead operating personnel where leakage presents a potential safety hazard. Use of separate pumps for each reagent and a separate mixing means adds to the fabrication and maintenance costs of the system and also increases the complexity of the controls required.